Phosphate composition and method for coating metallic surfaces



Aug. 20, 1963 E. R. REINHOLD PHOSPHATE COMPOSITION AND METHOD FOR COATING METALLIC SURFACES 3 Sheets-Sheet 1 Filed NOV. 16, 1960 INVENTO E J R- Rcmwwld g a: (12611 & ATTORQYSEYS M Aug. 20, 1963 E. R. REINHOLD 5 Sheets-Sheet 2 INVENTOR R Rambo A Mfligzhmx ATTORNEYS Aug. 20, 1963 E. R. REINHOLD PHOSPHATE COMPOSITION AND METHOD FOR CGATING METALLIC SURFACES I5 Sheets-Sheet 3 Filed Nov. 16, 1960 INVENTOR E R4 Relnho BY gT jLy- H 2, Kuhn.

ATTORNEYS United States Patent ffice 3,101,286 Patented Aug. 20, 1963 3,101,286 PHOSPHATE CGMPOSITION AND METHOD FOR COATING METALLIC SURFACES Earl R. Reinhold, Levittown, Pa., assignor to Arnchem Products, Inc., Ambler, Pa., a corporation of Pennsylvama Filed Nov. 16, 1960, Scr. No. @737 Claims. (Cl. 148-615) This invention relates to improved solutions for, and to an improved method of, coating the surfaces of zinccoated steel and iron to provide thereon an adherent, corrosion-resistant coating suitable as a base for protective finishing materials such as paint, lacquer, varnish, enamel, etc. In particular, the present invention relates to improved phosphate coating solutions and methods for their use in connection with certain types of hot-dipped galvanized iron and steel.

As will be pointed out more fully hereinbelow, the art of applying phosphate coatings to iron and zinc is Well developed. However, the products of certain types of high speed, continuous hot-dipped, zinc-coating processes such as those known commercially as Zincgrip or Ti-Co have proved unusually troublesome to the phosphate coating art. An example of such a zinc-coating process is disclosed in U.S. Patent 2,197,622. Briefly, the process comprises the steps of oxidizing the ferrous surface to form a thin oxide film thereon having a color ranging from light yellow to purple and ranging as far as gray, then reducing the oxide film to a film of the pure metal. While the reduced film remains protected in a non-oxidizing atmosphere, the surface, in a flux-free condition, is immersed in a molten zinc bath containing a minor quantity of aluminum to thereby form a zinc coating when the surface is withdrawn from the bath and allowed to cool.

Galvanized surfaces of the kind produced by the above process are particularly hard to coat by the well-known methods of the phosphate coating art. Even those solutions which have been found to produce satisfactory coatings on most zinc and zinc alloy surfaces have been found to be unsatisfactory for use on hot-dipped, zinc-coated material under consideration here. Such solutions when used on hot-dipped galvanized iron and steel produce coatings which are extremely non-uniform. The surfaces so produced are covered with multitudinous small raised areas where the coating action was apparently far more intense than on other areas of the surface. This phenomenon has been called seediness and is peculiar to the situation where phosphate coatings are applied to hotdipped galvanized surfaces.

Inasmuch as one of the principal objects of applying phosphate coatings to galvanized surfaces is to provide a suitable base for pain-ts, lacquers and the like, and in view of the fact that for many applications a rough uneven surface is undesirable, there is an acute need for a method and solutions for producing a satisfactory phosphate coating on this type of galvanized material.

U.S. Patent 1,221,046 discloses the coating of zinc and its alloys by treatment with a solution of phosphoric acid containing a nitrate accelerator. This patent and U.S. Patent 2,121,574 allege that more uniform phosphate coatings are obtained when nickel or cobalt ions are added to the phosphoric acid solution. U.S. Patents 1,869,121 and 1,888,189 disclose that copper ion in certain concentrations is beneficial in speeding up the action of a phosphate solution on a zinc surface, The surfaces of certain zinc alloys containing aluminum, which are coated only with great difficulty in nickel-accelerated phosphate solutions, are said in U.S. Patent 2,591,479 to be easily coated when fiuobor-ate ion is added to a phosphate solution in combination with nickel ions. All of these solutions are satisfactory in many applications, but it has been found that they all produce a seedy coating when used on hotdipped galvanized iron and steel.

U.S. Patent 2,835,617 discloses a method and solution for retarding seed formation in phosphate coatings by the introduction of soluble silicon containing ions, in particular, silicotluoride ions into the coating solution.

I have found that greatly improved phosphate coatings, which are corrosion resistant, firmly adherent, and very satisfactory as a base for paint and other finishes, are produced when soluble titanium ion is included as an essential ingredient in the phosphate coating solutions. The improvement in the coatings produced by my inven tion over those produced by solutions known in the art is so striking as to amount to a difference in kind rather than mere degree, for not only is the seediness which renders most prior art coatings unsatisfactory as paint bases eliminated, but the grain size of the coatings is modified, thereby improving the paint bonding properties. When paint is applied over surfaces which have been treated by the solutions and method of my invention. there results a satisfactory smooth surface, while the solutions of most of the prior art result in a product in which it appears that the paint has been applied over a layer of sand or grit.

While the solutions and methods of my invention are particularly useful in the coating of hot-dipped galvanized metals, they are also of great utility in the coating of other types of zinc-coated, ferrous surfaces such as electrogalvanized zinc surfaces.

It is an object of this invention to provide solutions and methods for phosphate coating hot-dipped, zinccoated iron and steel and other zinc surfaces.

It is an important object of my invention to provide solutions and methods for improving the paint bonding and corrosion resistant properties of hot-dipped, zinccoated surfaces.

A further important object of my invention is the provision of methods and solutions for the production of seedfree phosphate coatings on hot-dipped, zinc-coated surfaces.

Still another object of my invention is the provision of solutions and methods for phosphate coating zinc and zinc-coated surfaces to provide coatings wherein the grain size is adjusted to provide optimum paint bonding properties for the particular paint or other siccative coating which is to be applied.

The coating solutions of this invention comprise generally aqueous solutions containing phosphate ion, an ion such as zinc or manganese in suflicient proportions to produce a dihydrogen phosphate with the phosphate ion, a small proportion of nickel ion, at least a small amount of soluble titanium preferably introduwd as a titanium fluoridecontaining ion such as may be derived from fiuotitanic acid, ammonium fluotitanate, etc. and an oxidant such as nitrite or nitrate ions. In addition, the solution may contain a small amount of copper as an additional accelerator, if desired.

In the solutions of my invention the phosphate ions should be present in an amount from about 0.5 to about 2.5% (weight/volume). Associated with the phosphate ions are zinc or manganese ions in an amount at least sufficient to form the dihydrogen phosphate with the phosphate ion. It has been found, however, that if the proportions of zinc or manganese in the solutions are too high, seed-free coatings cannot always be obtained. Therefore, it is preferred that the solutions contain less than about 0.4 to 0.5% zinc or manganese. It is also preferred that the ratio of phosphate ion to zinc or manganese ion not be allowed to exceed 5:1.

An oxidizing ion such as nitrate or nitrite ion or a mixture of nitrate or nitrite ions is also present in the solutions of the invention. The quantity of nitrate ion d which is desirable is about 0.2 to about 1.0%, with major benefits being obtained at about 0.3 to about 0.5% nitrate ion. Substantially smaller amounts of nitrite ion are necessary to produce the desired results, hence the acceptable range for nitrite ion is from about 0.0002 to about 0.008%. I have found that if the concentration of the oxidant is too high, it is not always possible to obtain seed-free coatings (a situation similar to that noted with the zinc and manganese ions). Therefore, I prefer to use less than about 0.5% nitrate ion or less than about 0.005% nitrite ion. I find that it is also preferable to utilize solutions which have a total acidity of less than 40 points (see Example I for method of determining total acidity).

To summarize my preferences with regard to zinc and manganese ions and oxidant ions, in order to maximize the seed elimination characteristics of the solutions, I prefer that the zinc or manganese concentration be less than about 0.4 to 0.5% with a phosphate to zinc or manganese ratio of not more than :1 and that the oxidant concentration be less than about 0.5% nitrate ion or less than about 0.005% nitrite ion.

The maximum benefit from the use of nickel ion, and copper ion (if utilized), lies in a critically small range, between about 0.01 and 0.4% for nickel and between about 0.0003 and 0.005% for copper. Since the higher concentrations of these ions, especially of copper, seem to favor increased seediness, it is preferred to utilize more moderate amounts of these substances, for example, less than 0.3% nickel and as little copper as is consonant with satisfactory coating rates, proportions of 0.0003 to 0.001% usually being most satisfactory.

As has been mentioned, the inclusion of soluble titanium ions in the solutions of my invention improves the resultant coating in two respects, namely, the elimination of seeding and reduction in the size of the surface grains of the coating. The concentration of soluble titanium necessary for smooth, hard, and spot-free phosphate coatings is quite small. For example, as little as 0.009% titanium (supplied by ammonium fiuotitanate in the phosphating bath) is capable of producing spotfree coatings on hot-dipped galvanized surfaces. However, at this low concentration the reduction in grain size or grain refinement effected by the titanium is minimal. When the concentration is increased, until it approaches and exceeds about 2.5 grams per liter of soluble titanium expressed as ammonium fiuotitanate, visible grain refinement is brought about. A further increase in the concentration of soluble titanium results in increased grain refinement, that is, greater reduction in the size of the grains. At a concentration approaching the solubility limits of the soluble titanium compound, for example, approximately grams per liter for ammonium fluotitanate, the grain refinement is so pronounced that the grain structure loses most of its crystallinity and appears to become almost amorphous.

A concentrate which is suitable for dilution with water, as hereinafter described, to produce the coating solutions of my invention, and which is also suitable for addition to partially exhausted solutions as a replenisher, was prepared from the following materials.

Example I Percent by wt. Zinc oxide 8.0 Phosphoric acid (75%) 39.5 Nickel carbonate Ni) 3.0 Nitric acid (38 B.) 8.0 Hydrofluoric acid 1.0 H TiF (56%) 1.0 Water 39.5

The above formula when diluted with water to a strength of 4% by volume has the following analysis:

Total acidity 30.6 Free acidity 8.4 Percent nickel 0.075 Percent titanium 0.009

The total acidity given above is the milliliters of tenth normal sodium hydroxide (points) required to titrate a 10 ml. sample of the bath to a phenolphthalein endpoint, whereas, the free acidity is the mls. of tenth normal hydroxide (points) required to neutralize a 10 ml. sample of the bath to a brom cresol green endpoint. (It should be noted that soluble copper compounds may also be a component of a concentrate of the type illustrated in Example I, if it is desired to have copper present in the treating bath.) This diluted solution when heated '00 temperatures between about F., and preferably between F. and F., has been found to produce excellent, fine grain, seed-free, phosphate coatings on a variety of zinciferrous surfaces, especially hot-dipped galvanized steel panels.

Vwhcn similar hotdipped, zinc-coated, steel panels were coated in a bath similar to that of this example, except for the substitution of approximately equivalent amounts of hydrofluoric acid or hydroiluoooric acid for the soluble titanium, the phosphate coatings were objcctionably seedy. It is clear, therefore, that it was the soluble titanium and not the fluoride content that eliminated the scediness.

With respect to the conditions in the treating operation other than temperature, which has been discussed above, I have found that the concentrate of Example I may be diluted to a strength of about /2 to about 6% by volume with water; the preferred dilution being from about 1 to about by volume. Any conventional method of contacting the surface with the solution is satisfactory. The treating time (or contact time) may vary from about 15 seconds to about 10 minutes, but I prefer treating times from about 45 seconds to about 2 minutes.

After the treatment, the surface should be rinsed with water or with one of the final rinses which are customary in the art. Examples of such rinses are dilute solutions of chromic acid or phosphoric acid.

The concentrate of Example I (or a similar one containing copper in addition) when employed as a replenishing material for partially depleted solutions restores not only the desired level of the coating producing ingredients, but also the accelerators such as nickel and copper.

In the solutions of my invention the nickel and copper ions can be provided by adding a salt which is soluble in the solution, for example, a carbonate, nitnate, chloride or sulfate. Likewise, the soluble titanium is added to the solution in the form of an acid or salt of an acid such as iluotitanic acid or a stable soluble salt such as the sodium and potassium and ammonium salts thereof or potassium titanium oxalate.

The novel seed elimination action and grain refinement action of my invention are effectively demonstrated by the following example. A concentrated solution consisting of:

Example 11 Percent by wt.

H P0 (75%) 39.5 ZnO 10.0 NiCO (45% Ni) 3.0 NHO 42 B.) 6.0 HP 49.1% 2.0

Water, to make 100.0 percent by wt.

was prepared. This solution was subsequently diluted to 4% by volume with water and 5 aliquot samples of the diluted solution were prepared.

No soluble titanium compound was added to the first solution. One gram per liter of soluble titanium in th form of ammonium fluotitanate was added to the second solution. Similarly, 2 /2, and 25 grams per liter of ammonium fluotitanate were added to the rwpective remaining solutions. The solutions were then brought to a temperature of approximately 150 F. Each solution is sprayed onto one of five alkali cleaned, hot-dipped galvanized panels for a period of one minute. After being sprayed, each panel was dried and ph-otomicrographed at 100x. FIGURE 1 is a photomicrognaph at the above magnification of the panel treated with the solution containing no soluble titanium. FIGURES 2, 3, 4 and 5 are photomicrographs at the same magnification of panels sprayed in solutions containing 1, 2% 5 and 25 gnarns per liter respectively of ammonium fiuotitanate.

'Ilhe photomicrograp hs do not reveal the three-dimensional nature of the surface coating. However, a comparison of FIGURES l and 2 reveals that, although the size of the crystals or gnains is approximately the same, the crystals in FIGURE 1 are more mu together and unevenly distributed across the surface than those of FIGURE 2. In other words, the coating of FIGURE 2 appears to be very even, Whereas that of FIGURE 1 is quite uneven.

When a comparison is made between FIGURES 2, 3, 4 and 5, it can be seen that with each succeeding increase in the concentration of the soluble titanium bearing ion, the size of the grains or crystals is reduced. In FIGURE 5, which is a photomicrograph of the panel treated with the most concentrated titanium bearing solution, the grain refinement is so pronounced that it is difiicult to trace any pattern of crystallinity. The surface appears, even in the photomicrognaph, to be almost amorphous.

Where brittle paint films are being employed and high covering power with minimal paint thickness is desired, in general, it is preferred to use coatings 'which have a high degree of grain refinement. Where substantial amounts of paint or siccative coating are desired on the surface, the increased Working action of the larger crystal structure is desirable [so smaller amounts of soluble titanium will probably be desired. In all events, as may be seen from the photomicrographs appearing above, the desired degree of crystallinity and particle size of the conversion coating on the surface of the metal can be modified to meet with the users specific needs at the moment.

I claim:

1. An aqueous solution for coating a continuous hotdipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5 to 2.5 phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least suflicicnt to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, and a titanium containing ion in a proportion sufficient to produce at least 0.009% soluble titanium.

2. An aqueous solution for coating a continuous hotdipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5 to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sulficient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, from about 0.0003 to about 0.005% copper ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, and a titanium containing ion in a proportion suflicient to produce at least 0.009% soluble titanium.

3. An aqueous solution for coating a continuous hot dipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5 to 2.5 phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least suflicient to form dihydrogen phosphate with said phosphate ion, to not more than 0.5% metal ion, a ratio of phosphate ion to said metal ion of not more than 5:1, 0.01 to 0.3% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.3 to 0.5% nitrate ion and 0.0002 to 0.005% nitrite ion, and a titanium containing ion in a proportion sufficient to produce at least 0.009% soluble titanium.

4. An aqueous solution for coating a continuous hotdipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5% to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, and from about 1 gram per liter to about 25 grams per liter of soluble titanium expressed as ammonium fiuotitanate.

5. An aqueous solution for coating a continuous hotdipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5% to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufiicient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, and fluotitanic ion in a proportion sufiicient to produce at least 0.009% soluble titanium.

6. An aqueous solution for coating a continuous hotdipped, zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5 to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with said phosphate ion, to not more than 0.5% metal ion, a ratio of phosphate ion to said metal ion of not more than 5 :1, 0.01 to 0.3% nickel ion, from about 0.0003 to about 0.001% copper ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.3 to 0.5 nitrate ion and 0.0002 to 0.005% nitrite ion, and fluotitanic ion in a proportion sufficient to produce at least 0.009% soluble titanium.

7. In the art of phosphate coating continuous hotdipped, zinc-coated, ferrous surfaces by contacting said surfaces with aqueous solutions the coating-producing ingradients of which consist essentially of about 0.5% to 2.5 phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufiicient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, the improvement which comprises including in said solutions a titanium containing ion in a proportion sufficient to produce at least 0.009% soluble titanium.

8. The method of providing seed-free phosphate coatings on continuous hot-dipped, zinc-coated, ferrous surfaces comprising contacting the said surfaces with an aqueous solution the coating-producing ingredients of which consist essentially of about 0.5 to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in aconcentration of about 0.2 to 1% nitrate 7 ion and 0.0002% to 0.008% nitrite ion, and a titanium containing ion in a proportion sufficient to produce at least 0.009% soluble titanium.

9. The method of providing seed-free phosphate coatings on continuous hot-dipped, zinc-coated, ferrous surfaces comprising contacting the said surfaces which an aqueous solution the coating-producing ingredients of which consist essentially of about 0.5% to 2.5% phosphate ion, a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufiicient to form dihydrogen phosphate with said phosphate ion, 0.01 to 0.4% nickel ion, from about 0.0003 to about 0.005% copper ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.2 to 1% nitrate ion and 0.0002% to 0.008% nitrite ion, and a titanium containing ion in a proportion sufficient to produce at least 0.009% soluble titanium.

10. An aqueous solution for coating :1 zinc-coated, ferrous surface the coating-producing ingredients of which consist essentially of about 0.5% to 2.5% phosphate ion,

a metal ion selected from the group consisting of the zinc ion and the manganese ion in a proportion at least sufficient to form dihydrogen phosphate with said phosphate ion, to not more than 0.5% metal ion, a ratio of phosphate ion to said metal ion of not more than 5:1, 0.01 to 0.3% nickel ion, at least one oxidizing ion selected from the group consisting of the nitrate ion and the nitrite ion in a concentration of about 0.3 to 0.5% nitrate ion and 0.0002 to 0.005% nitrite ion, and fluotitanic ion in a pro portion sufficient to produce at least 0.009% soluble tita mum.

References Cited in the file of this patent UNITED STATES PATENTS 2,516,008 Lum July 18, 1950 2,813,812 Somers et al Nov. 19, 1957 FOREIGN PATENTS 757,050 Great Britain Sept. 12, 1956 

1. AN AQUEOUS SOLUTION FOR COATING A CONTINUOUS HOTDIPPED, ZINC-COATED, FERROUS SURFACE THE COATING-PRODUCING INGREDIENTS OF WHICH CONSIST ESSENTIALLY OF ABOUT 0.5% TO 25% PHOSPHATE ION, A METAL ION SELECTED FROM THE GROUP CONSISTING OF THE ZINC ION AND THE MANGANESE ION IN A PROPORTION AT LEAST SUFFICIENT TO FORM DIHYDROGEN PHOSPHATE WITH SAID PHOSPHATE ION, 0.01 TO 0.4% NICKEL ION, AT LEAST ONE OXIDIZING ION SELECTED FROM TH GROUP CONSISTING OF THE NITRATE ION AND TH NITRITE ION IN A CONCENTRATION OF ABOUT 0.2 TO 1% NITRATE ION AND 0.0002% TO 0.008% NITRITE ION, AND A TITANIUM CONTAINING ION IN A PROPORTION SUFFICIENT TO PRODUCE AT LEAST 0.009% SOLUBLE TITANIUM. 